Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count drm_gpuvm structures

[Christian König]

Am 10.11.23 um 17:57 schrieb Danilo Krummrich:
> On 11/10/23 09:50, Christian König wrote:
> > [SNIP]
> > > > > Another issue Christian brought up is that something intended to 
> > > > > be embeddable (a base class) shouldn't really have its own 
> > > > > refcount. I think that's a valid point. If you at some point need 
> > > > > to derive from multiple such structs each having its own refcount, 
> > > > > things will start to get weird. One way to resolve that would be 
> > > > > to have the driver's subclass provide get() and put() ops, and 
> > > > > export a destructor for the base-class, rather than to have the 
> > > > > base-class provide the refcount and a destructor  ops.
> > >
> > > GPUVM simply follows the same pattern we have with drm_gem_objects. 
> > > And I think it makes
> > > sense. Why would we want to embed two struct drm_gpuvm in a single 
> > > driver structure?
> >
> > Because you need one drm_gpuvm structure for each application using 
> > the driver? Or am I missing something?
>
> Right, *one*, but not more than one. Wasn't that the concern? Maybe I 
> misunderstood something. :)

Well, there is the use case of native context with XEN/KVM. In that 
situation QEMU opens tons of driver file descriptors on behalves of the 
virtual environment clients.

In this use case you have many drm_gpuvm instances for a single 
application. So you can't assume that you only have one VM per PID/TGID 
or something like that.

AMD already made that mistake with KFD and I strongly suggest not to 
repeat it :)

Regards,
Christian.